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The QRD Diffractal: A New One- or Two-Dimensional Fractal Sound Diffusor

Number theoretic diffusors, as described by Manfred Schroeder, are characterized by a periodic grouping of a series of wells of equal width, but different depths, separated by thin dividers. Over the past seven years we have found the quadratic residue depth sequence very useful in hundreds of applications, including recording studios, concert halls, worship spaces, conference rooms, music education and even home listening rooms. The frequency bandwidth is determined principally by the depth of the deepest well, which determines the low frequency limit , and the well width, which determines the upper limit. Physical manufacturing constraints pose a limitation of approximate 1" on the well width and approximately 16" on the depth, after which the units become diaphragmatic and defeat the purpose of increasing the depth further. To extend the low frequency range and bandwidth of the QRD trademark, we have developed a fractal diffusor, the DIFFRACTAL trademark, which is in effect a diffusor within a diffusor, each covering a different frequency range, much like a multi-way loudspeaker. The full spectrum bandwidth and cross-over points are completely calculable. The mid-high frequency diffusors are placed on the wells of a dedicated stiff and massive low frequency diffusor. These bandlimited nested diffusors allow virtually any bandwidth, limited only by the available depth and the surface detail, and aesthetic design. We will describe two applications of the DIFFRACTAL trademark at Peter Gabriel's Real World Studios, Bath, England and INXS' Rhinoceros Recording Studios, Sydney, Australia. We will also describe a two dimensional DIFFRACTAL capable of providing even further low frequency control.